T0C04: What factors affect the RF exposure of people near an amateur station antenna?
A. Frequency and power level of the RF field
B. Distance from the antenna to a person
C. Radiation pattern of the antenna
D. All of these choices are correct
Radio frequency exposure is usually not a significant hazard for most amateur radio stations. However, you should be familiar with the FCC recommended Maximum Permissible Exposure (MPE) limits, know how to evaluate your station for RF exposure limits, and know how to keep exposure levels safe by making adjustments to your station. Section 13.3 of the HamRadioSchool.com Technician License Course book, RF Exposure Safety, has an excellent discussion on these issues and practical advice on how to easily evaluate your station. You will also find links to more RF safety information in the online Technician Section Media for 13.3.
What makes excessive RF exposure dangerous? Radio frequency radiation is not ionizing radiation. This means that RF does not strip electrons from atoms leaving electrically charged particles and it does not alter DNA genetic molecules. Rather, RF energy is absorbed by our body’s tissues and the result is heat. Like that leftover chunk of ham in the microwave oven, our body tissues can get warmed up by absorbing RF energy. And some parts of our bodies are more susceptible to this kind of heating than others because of a relatively reduced capacity to efficiently carry away the heat energy. Our eyes are one notable example. Too much RF for too long of a period can result in enough heating that some tissues may be heat damaged.
Our body absorbed some radio frequencies more readily than others. The most easily absorbed frequencies are right smack in the Technician privileges, too – the VHF range, from 30 – 300 MHz. This includes the very popular 2-meter and 6-meter amateur bands. So, the lowest values for recommended MPE are associated with the VHF bands. So, for equal power levels you’ll get a bit warmer due VHF exposure than from HF or UHF exposure. Frequency matters!
Still, for most stations these effects from VHF are not that big of a worry. In fact, your station may radiate up to 50 watts (PEP at the antenna) in the VHF frequencies before an exposure evaluation is required. While you still don’t want to straddle your antenna while it’s transmitting or gaze lovingly at it from mere inches, you are not likely to receive significant tissue warming in typical operating cases such as an elevated exterior antenna, a mag-mount car antenna, or even a cookie-sheet mounting antenna inside your house up on a bookshelf. The 5 watts of your HT is perfectly safe, even at very close range. But, if you’re pumping out more than 50 watts, you are required to evaluate your station’s exposure. Power matters!
The distance of a human from the radiating antenna significantly affects exposure. The intensity of RF radiation falls off as the square of the distance from the radiating element. So, double your distance from the antenna and you have reduced your exposure to ¼ the previous level. Relocating an antenna is one of the most common actions to take to prevent exposure to RF radiation in excess of FCC-supplied limits. Distance matters!
The gain of an antenna can also impact exposure. Reference Chapter 7, Antennas, to learn how directional or “beam” antennas direct most of the radiated energy in one direction, thereby changing the radiation pattern as compared to an antenna that radiates equally in all directions (isometric antenna). When you collect all that RF and push it in one direction the RF intensity in that aimed direction is increased, and this gain in signal strength must be considered in evaluating your station’s situation. Radiation patterns matter!
So, to reduce the RF exposure to humans caused by your station you may want to do any of the following:
- Relocate your antenna away from people or extend it higher
- Reduce your average transmitter power, especially in the VHF bands (lower PEP and/or duty cycle)
- Use frequencies outside of the VHF band
- Point your directional antenna away from people
The answer to Technician pool question T0C04, “What factors affect the RF exposure of people near an amateur station antenna?” is D: All of these choices are correct.